Yet, proponents say, relatively pure nitrogen is a superior medium for inflating tires compared to air. It's a naturally occurring inert gas making up roughly 78 percent of the air we breathe - the same air we compress to inflate tires. If it's so innocuous, how does it produce the fuel economy and tire life benefits we hear about?

First, a little clarification. You're not putting anything new into your tires; nitrogen is already there in air - along with about 21 percent oxygen and 1 percent trace elements. The so-called nitrogen generators used in tire inflation systems don't make nitrogen; they use a membrane process to remove most of the oxygen from the air, leaving you with an inflation medium that is 95 to 98 percent pure nitrogen.

The key benefit to nitrogen tire inflation is that it slows the natural pressure loss in tires. Oxygen is able to permeate tire rubber, producing the expected 1 to 5 percent loss of inflation pressure over a month or so. Nitrogen permeates more slowly because nitrogen molecules are physically larger than oxygen molecules. Barring leaks, poorly seated beads, bad valves, etc., a tire inflated with nitrogen will retain a constant pressure longer than one filled with air, minimizing the headaches caused by under-inflation or mismatched inflation across sets of duals.

ADVERTISEMENT

Additionally, nearly pure nitrogen is devoid of moisture, so you won't have water sloshing around in the tire, possibly corroding steel wheels, or freezing tire valves in winter. It is also believed that as it passes through the walls of the tire, oxygen causes minor degradation of the liner and casing rubber through oxidation.

We contacted three fleets who use nitrogen to inflate their tires, and two out of three said they are very pleased with tire performance. The third was equally pleased, but said they got the same results using an automatic tire inflation system.

"We used the nitrogen system for about a year, and it worked just the way they told us it would," says Chuck Frost, maintenance manager for Gypsum Express in Syracuse, N.Y. "We weren't dissatisfied with it, but we found PSI's automatic tire inflation system to be more cost-effective."

Paul Mincy of Adams Motor Express in Carnsville, Ga., on the other hand, swears by nitrogen.

"We're running a five-to-one trailer-to-tractor ratio, and lots of those are dropped trailers. Some are away from the shop three months at a time, but we've seen little if any pressure loss on those tires," he says. "We've got 2,800 tires on nitrogen now, and we haven't had a soft-tire blowout in four years. The year before we started we had 63 pressure-related failures."

Mincy says he hasn't even tried to calculate the fuel economy benefits because there are too many other variables to worry about here, but he says tread wear has improved, and time spent checking pressure and airing up tires has all but disappeared from his billing roster.

"Our pressure compliance is close to 100 percent now, at no additional cost to the fleet for drivers' or shop time to maintain pressure," he says. "Some fleets might not notice, or might expect too much from the system, but a fleet with a good tire tracking program will see the advantages very quickly."

Walmart is such a fleet. That company currently has nitrogen inflation systems installed at more than 50 of its fleet service centers. Walmart does not discuss its maintenance strategies publicly, but Chris Sultemeir, the retailer's senior vice president of transportation services, appears in a promotional video on Walmart's corporate website saying tire inflation is part of their fuel savings and emission reduction strategy.

"What we plan to do from a tire standpoint, and what we're currently testing, is nitrogen filled tires or an automatic filling process to maintain a constant air pressure," he says. "It's critical from the standpoint of rolling resistance with the tires that we keep a constant pressure in these tires for maximum fuel efficiency."

Paying for air?

On the surface, it might seem like you're paying a lot for air just to get nitrogen inflation. Compressed air is basically free and available anywhere, but the quality of that air and its moisture content is suspect at some locations. Shop air, generally, is clean and dry, but you have to invest a little money in a good shop air system. With nitrogen filling, you're investing a little more money to get a better quality inflation medium that, all things being equal, will alleviate some of the downstream grief associated with tire pressure maintenance - like pleading with drivers to check their tires, and paying technicians to fill tires that drivers didn't check.

Tire pressure monitoring systems will alert you to a low tire, but won't top it up. Automatic inflation systems will top up a tire for you, but won't necessarily tell you which tire is leaking - although a combination of the two can resolve that. Tire inflation systems can mask tire punctures if the driver isn't aware that the system is working, or it comes on only briefly. If a puncture exists, moisture can still seep into the casing, possibly compromising the steel cord.

It all comes down to the return on your investment. According to Bill Hyatt of Inflation Solutions Group, a capital investment of $45,000 amortized over a fleet of 200 trucks leads to a one-time cost of $225 per truck at start-up. How long it might take to recover $225 per truck from failed tires is the question. After that - and a little ongoing system maintenance - nitrogen, like air, is basically free.

The tire manufacturers do not endorse nitrogen as a fill medium, nor do they prohibit it. Several acknowledge the benefits of lower seepage rates relative to maintaining proper inflation pressure.

Fuel economy benefits from properly inflated tires are tangible, but nitrogen alone isn't the instrument of improvement. Nitrogen will slow a tire's rate of pressure loss relative to air, and thus maintain a set pressure for longer, but those tires can't be ignored. They can always suffer a puncture, and that will let the air out of your tire maintenance plans as quickly as anything else.